The invention is directed to a method and device for determining the degree of compaction during ground compaction by means of a vibrating plate compactor or a roller, comprising a top section and a vibrating bottom section and driven with a certain excitation frequency.
In carrying out ground compaction there is basically a desire to obtain a statement of the degree of compaction achieved at any time so as, on the one hand, to be able to guarantee the required compaction values while, on the other hand, obtaining the most efficient possible use of the compaction equipment. In particular, it is desired to cease compaction when further passes are no longer profitable or would even lead to re-loosening of the ground.
Consequently, numerous solutions are already known, involving measurement during the compaction process of certain vibration parameters, which are then used to determine the degree of compaction achieved. However, these systems are in practice suitable only for compaction rollers, and not for vibrating plate compactors. The reason for this is partly the high cost of the equipment, making it uneconomical for vibrating plate compactors, but partly also the much higher acceleration values of the vibration plates, which are around twice the level of those of vibration rollers.
From this starting point the problem of the present invention is to provide a system for determining the degree of compaction which is suitable, not only for rollers but also for vibrating plate compactors, is able to withstand the high acceleration values occurring with the latter, and is particularly distinguished by relatively favorable costs of production.
This problem is solved according to the invention by determining one or more amplitude values of the vibration of the bottom section relative to the top section at the excitation frequency, together with one or more amplitude values of one or more vibrations of the bottom section relative to the top section at a maximum of 60% of the excitation frequency, with the quotient of the aforementioned amplitude values then being used as a measure of the current degree of compaction of the ground.
Studies made by the applicant have revealed, surprisingly, that the quotient defined above rises continuously with the number of passes, and is a reliable indicator for firmness of the ground. As is usual, the value of this quotient depends heavily on the properties of the ground to be compacted and the compaction equipment used, but its relative change from one pass to the next indicates clearly to the operator whether the firmness of the ground has increased, and when further passes are no longer profitable or may even be adverse.
The major advantage of the system according to the invention lies in the fact that no absolute values need to be measured, but only the relative movements between top section and bottom section. These vibration amplitudes may be picked up from the top section without contact, in particular by inductive means. At the same time, no sensor need be attached to the vibrating weight, and problematic cable connections to the vibrating weight are avoided. A further advantage lies in the fact that the amplitudes may be separated according to their frequency relatively inexpensively by electronic means.
The solution according to the invention therefore stands out for its comparatively simple and inexpensive design and its high reliability.
For the amplitude values of the vibration occurring at a maximum 60% of the excitation frequency, it is recommended that a broad frequency band, ranging for example from about 1% to about 50% of the excitation frequency, be taken as a basis. This frequency band may then be utilized over its whole width, or just a relatively small frequency range extending for example from 10 Hz to 20 Hz may be picked out, or several narrow frequency ranges from the specified frequency band may be superimposed.
With regard to the amplitudes occurring at the excitation frequency, it is recommended that a fixed value be specified (preset) for the excitation frequency, i.e. to use the vibration frequency specified by the manufacturer of the compaction equipment as a basis, and to measure the amplitudes for this frequency. However, it is also within the scope of the invention to specify a variable value for the excitation frequency, in particular if the actual excitation frequency is unstable. Recommended in this case is the measurement of a value which is proportional to the excitation frequency. This measured value may then be used for signal filtering, so that the amplitude is measured in each case at the current excitation frequency. The amplitude values of the various frequency ranges may be determined by Fourier transformation, particularly by FFT (Fast Fourier Transformation).
In principle, the amplitude values determined and/or the quotient calculated from them should be averaged, since the signals fluctuate strongly. One measured value per second is quite sufficient. The averaging may be effected, for example, by using envelope curves.
So that the operator can recognize from what point onwards further passes are no longer profitable, a visual or audible signal is expediently generated when the aforementioned quotient passes a defined limit value or its rate of change is too low.
To implement the method described above, it is recommended that the top section have a sensor for non-contact detection of the relative movements between top section and bottom section, in particular a sensor for inductive data acquisition, corresponding to a measuring face lying opposite on the bottom section. This has the advantage that the sensor and its electrical connection are not exposed to the sharp accelerations and decelerations of the vibrating bottom section. The measuring device is therefore distinguished by good reliability and long life, and is especially suitable for vibrating plate compactors.
Preferably, a high-pass filter and a bandpass filter are used to separate the frequency components, with the high-pass filter separating the amplitude value of the vibration occurring at around excitation frequency, and the bandpass filter separating the amplitude value of the vibration occurring at a maximum 60% of the excitation frequency. Preferably, the bandpass filter allows the passage of amplitude values from a frequency range of about 1% to about 50% of the excitation frequency, in practice for example from 1 Hz to 30 Hz, when the excitation frequency is 60 Hz.
Naturally, this bandpass filter may also be replaced by a high-pass filter with a 1 Hz cutoff frequency and a low-pass filter with 30 Hz, connected in series.
For averaging, use may be made either of the amplitude values directly or of the quotients formed from them. In each case, a low-pass filter with a cutoff frequency of about 0.2 Hz to 1 Hz is used.